This invention relates to a tire pressure indicating system for on-board use in an automotive vehicle, such as a car, truck, or tractor-trailer, to convey inflation pressure information from the rotating pneumatic tires on which the vehicle travels.
A pneumatic tire for an automotive vehicle is designed for certain pressure and load range. While the manufacturers note the importance of having proper inflation and of not exceeding the specified load range, they themselves have no control over them once the tire goes into use on a customer's vehicle. In other words, after the sale it is the customer's responsibility to assure that overloading is avoided and proper pressure maintained.
Both overinflation and underinflation can have consequences detrimental to tire life and performance. Overinflation can be avoided by the exercise of due care at the time of filling the tires with air. The only way to guard against prolonged periods of underinflation however is by frequently checking the pressure, and replenishing the tire with air if underinflation is indicated.
The visual observation of a tire as an indicator of underinflation is not reliable, especially with today's tires. The manual checking of each of four tires on an automobile by means of a tire pressure gauge is obviously more accurate than visual observation, but involves more than a nominal expenditure of effort. For each tire, the valve cap must be removed, the tire pressure gauge pressed against the exposed end of the valve stem to obtain a reading, the tire filled if the reading indicates too low a pressure, and finally the valve cap replaced. It is unrealisitic to expect that the typical vehicle owner will perform this degree of maintenance on a frequent basis. At best, tire pressure checking will be done randomly, if at all.
Moreover, when the weather is inclement and/or the temperature cold, the probability of tire pressure checking is even less; yet these are the times when proper tire pressure can be most important. For example, a properly inflated tire in summertime temperatures will become underpressured in wintertime temperatures due to Boyle's Law. It is probably not an unreasonable statement that most vehicles in use today have one or more underinflated tires.
The owners and operators of commercial rigs and fleets are affected on a much larger scale by underinflated tires, and hence, they may be prone to perform tire pressure checking on their rigs and fleets more regularly than most people do on their own personal vehicles. In the course of such checking by fleet and rig owners, there will doubtless be many tires which when checked are found to be properly inflated, and therefore the time involved in checking these tires is an extra expense to the fleet and rig owners which could have been avoided if it were aforeknown that the pressure would be proper.
Patent literature is replete with diverse means and methods for indicating tire pressure without uncapping the valve stem and taking a measurement with a tire pressure gauge. Some relate to on-board pressure measurement where an occupant of the vehicle is informed if a tire is underinflated. On-board systems are advantageous if they can provide information concerning the present status of the tires, thereby presenting the driver with the opportunity to detect underinflation in its incipiency, and consequently the opportunity for taking corrective action much sooner than might otherwise be the case. Many of these prior schemes are too elaborate, too complicated, or too expensive to be practical. Some give an indication only when the pressure drops below a threshhold, and are incapable of sensing over a range of pressures.
Apart from the obvious safety implications of underinflated tires, it is well documented in the industry that underinflated tires wear at significantly higher rates than those properly inflated. The economic loss to the public due to the premature wear which accompanies underpressurized tires would likely surprise many people.
A commercially acceptable on-board tire pressure indicating system should alert vehicle operators to incipient underinflation conditions, giving them the opportunity to avoid the above-mentioned problems incidental to underinflation. The deployment of such a system would also eliminate the need to manually check the individual tires in the manner described above. This would be a convenience for the owners of personal cars and trucks. For fleet and rig operators, not only would it afford savings from the standpoint of reduced premature tire wear, but also it would avoid the time and expense in checking tires which are already property pressurized.
One of the serious impediments to a successful system involves the transmission of the tire pressure information from the rotating tire and wheel. Prior proposals include the use of slip rings, radio frequency transmitters, and magnetic pick-ups. Each of these has its own inherent problems, and none has received widespread commercial acceptance, if indeed any at all. The applicant is unaware of any commercial systems in existence today.
The present invention relates to a new and unique system for on-board tire pressure indication which is practical for mass-production usage both as original equipment and as an aftermarket product. Large scale acceptance of an on-board system mandates that the cost be reasonable, the installation not overly difficult, and that in operation the system be accurate and reliable. The present invention meets all of these requirements.
The invention possesses a number of unique features. One is the sensor for deriving the tire pressure information from the rotating wheel and tire. Another is the means by which information from the sensor is processed into a meaningful form.
For each tire of the vehicle, there is a receiver which is disposed adjacent the rotating wheel on which the tire is mounted. The preferred receiver is a Hall sensor. On the wheel is a transmitter which comprises three magnets. The preferred magnets are samarium cobalt. The magnets are spaced apart in a general circumferential sense on the wheel, and lie on essentially the same radius. As the wheel rotates, the magnets repeatedly sweep past the Hall receiver which detects the passage of each magnet.
Two of the three magnets are disposed in a predetermined fixed circumferential spacing on the wheel. The third magnet is selectively position in a circumferential sense relative to the other two magnets in accordance with the pressure in the tire. Hence, the distance between the two fixed magnets represents a reference measurement dimension, and the distance between the third magnet and one of the two fixed magnets represents an inflation pressure measurement dimension.
The sensor detects the passage of the three magnets by a time sequence of three pulses. The time between the two fixed magnet pulses corresonds to the reference measurement dimension while the time between the selectively positioned magnet's pulse and the one of the two fixed magnet's pulse corresponds to the inflation pressure measurement dimension. The ratio of the two measurement dimensions is essentially independent of the rotational speed of the wheel and tire.
The actual reference measurement dimension on the wheel is constant, but the sensed reference measurement dimension, in terms of time interval between the two fixed magnets' pulses, will vary as a function of the rotational speed of the wheel and tire; specifically being inversely proportional to speed. The actual inflation measurement dimension on the wheel is constant for a given inflation pressure, and in analogous manner to the sensed reference measurement dimension, the sensed inflation measurement dimension as measured by the time between the one fixed magnet's pulse and the selectively positioned magnet's pulse will be inversely proportional to rotational speed. The ratio of the sensed reference measurement dimension to the sensed inflation measurement dimension is essentially speed insensitive because velocity factors out when the two sensed measurements are ratioed.
The signals from the sensor are supplied to electronic circuitry which performs the ratio of the sensed inflation measurement dimension to the sensed reference measurement dimension, and it is this ratio which is indicative of the actual inflation pressure. Different pressures produce different positioning of the positioned magnet whereby the sensed pressure measurement dimension is correlated with pressure. Stated another way, the sensed pressure measurement dimension represents the pressure measurement dimension uncorrected for velocity, and the sensed reference dimension measurement provides the velocity correction factor which is used by the electronics to correct the former measurement dimension.
The electronics operates on a multiplex basis, sampling each wheel sensor in a repetitive sequence, and the inflation pressure information from each tire is obtained very quickly.
The three magnets of each transmitter lie within a limited angular extent of the corresponding wheel's circumference. Hence, there is a relatively long interval between passages of each transmitter past the corresponding receiver. The correct sequence of the magnets moving past a receiver is established by using the lack of signals during this relatively long interval to automatically reset the electronics, a further feature. Moreover, typical change in wheel velocity occurring during passage of the transmitter past the receiver will not have any significant detrimental effect on the relative measurement dimensions detected by the sensor, and therefore, not adversely affect the accuracy.
A preliminary novelty search in connection with this invention revealed only one patent which embodied a Hall-type device in a tire pressure indicator system. U.S. Pat. No. 4,330,774 discloses a tire pressure sensing system in which a Hall effect switch is used in a sensor. A single magnet is positioned axially of the wheel in accordance with tire pressure. The magnet is disposed at the same radius as the Hall effect switch to sweep past the switch during each revolution of the wheel. So long as the pressure is above a certain level, the magnet is sufficiently retracted not to activate the switch; however, below that pressure level, the magnet is sufficiently extended enough to activate the switch causing an alarm to be given. Although the present invention uses Hall sensors, it is conceptually different from and superior to the system proposed in U.S. Pat. No. 4,330,774.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.